1. Field of the Invention
The present invention relates motor vehicle wheels which is capable of withstanding the load even at low or zero pressure.
2. Description of the Related Art
It is well known that, since tyres were first developed, a large number of solutions have been proposed in an attempt to solve the problem relating to travel of said tyres in a deflated or—to use the technical term—“flat” condition. In fact this problem is important not only in the event of punctures during normal travel, but also with regard to military vehicles and therefore punctures caused for example by bullets.
Without mentioning, for the sake of brevity, the proposed solutions based on the use of sealants of various types, mechanical devices which must be mounted on the rims, special rims, sponge-like inserts, etc., it is sufficient to consider the structures of tyres which are self-supporting, namely are able to withstand the load even under zero inflation pressure, in order to find in the prior art a large number of solutions which, however, hitherto have not produced satisfactory results.
In this connection it should be stated first of all that, in a tyre traveling in a straight line, the only element which is subject to compression is the air contained inside it, whereas the tyre as a whole may be regarded as a tensile structure, i.e., in which all the structural elements which support loads work under tension.
In the case of a tyre, compressive stress is understood as meaning a stressed state such that the hydrostatic component of the tensor of the forces and the tensor of the deformations has a negative value.
It is obvious that when there is no internal-air pressure, if the structure is to continue withstanding the load applied, it must necessarily comprise elements which work under compression.
In general these elements, when they are made of elastomer material, involve not only an obvious increase in the weight of the tyre, but also an appreciable increase in the absorption of power due to elastic hysteresis and therefore in the rolling resistance of the tyre and its working temperature.
Among the solutions of the known art which can be associated with this approach, the following may be mentioned:
U.S. Pat. No. 5,238,040 envisages a structure of the tyre sidewall which includes three lens-shaped inserts, in addition to the reinforcing cords which are subject to tractional stress. The inserts in question are subject to complex stressed states which include a negative hydrostatic component or compressive component.
The rolling resistance of this tyre, although being less than that of other tyres which are based on the same principle (for example consisting of a single lens-shaped element reinforcing the sidewalls of the tyre), remains appreciably greater than the rolling resistance which is typical of normal tyres.
U.S. Pat. No. 3,708,007 describes a tyre containing two annular reinforcing elements which are inserted in the fold of a belt layer and which have the function of improving the performance of the tyre during driving of the vehicle. From the description and from the drawings it may be understood that the two annular elements are associated with a traditional geometry of the tyre, so that “flat travel” cannot be ensured and moreover is not mentioned even hypothetically in the text of this patent.
A structure comprising two cords, which in this case also are inserted at each end of the belt in a fold thereof, is described and illustrated in U.S. Pat. No. 3,831,657. The two cords in question do not have a structural function, but only that of facilitating folding of the ends of the belt during the tyre manufacturing process, as moreover confirmed by the suggested use of two nylon cords, which are entirely incapable of withstanding compressive forces and also very ineffective when subjected to tractional forces.
The tyre described and illustrated in U.S. Pat. No. 4,307,767 is specifically intended for travel at zero pressure and has two sidewalls which are devoid of a textile reinforcement and have an inverted curvature, namely a concave external surface and convex internal surface, so that they work exclusively under compression in any travel condition and therefore both during normal travel and during “flat travel”, namely at zero internal pressure.
Between the sidewall and crown of the tyre, owing to the specific curvature of the sidewalls, an imperfect hinge is formed, said hinge, when subject to the flexural forces of normal travel, giving rise to the risk of rupture due to fatigue. For this reason the tyre has two annular reinforcing elements consisting of two metal bead cores which are located in the shoulders of the tyre and are capable of imparting radial rigidity, under compression, to the structure of the sidewalls.
In the event of flat travel, the contribution in terms of rigidity which can be attributed to annular reinforcing elements of this kind is marginal compared to the contribution offered by the compressive resistance of the sidewalls.
In short the tyre described in this prior patent bases its load-bearing capacity on the compression of the sidewalls, both when the tyre is inflated and during flat travel.
U.S. Pat. No. 5,685,927, specifically relating to flat travel, describes a composite reinforcing structure which envisages, on the one hand, a plurality of elastomer inserts which are situated in the side and, on the other hand, at least one bead wire passing along the middle plane between the carcass plies and the tread band. This consists of a complex structure with a high manufacturing cost.
An alternative solution, which is envisaged in many prior patents, is that consisting in an annular structure reinforcing the crown of the tyre (as, for example, in U.S. Pat. Nos. 3,734,157, 4,111,249, 4,428,411, 4,459,169, and 4,673,014), a structure which occupies substantially the entire width of the tread band and reinforces flexurally the belt structure. In the case of the solutions proposed in the past with regard to self-supporting structures suitable for flat travel and involving the presence of elastomer structural elements, intense overheating occurs both during normal travel and, even more so, during flat travel.
This overheating in turn causes thermal/oxidative degradation of the materials, in particular the elastomer structural elements, and the consequent defects.
Moreover, in order to ensure the load-bearing capacity during flat travel, hitherto it has been considered indispensable to use bending-resistant or compression-resistant elements, attaching less importance to or even ignoring the thermo-chemical aspects and the problems associated therewith.
Although alternative solutions (previously mentioned) are known where the entire periphery of the tyre is reinforced and overheating is limited, the performance and comfort as well as duration of the tyre are greatly affected as a result during normal travel.